Abstract
Understanding how droplet condensation happens plays an essential role for our fundamental insights of wetting behaviors in nature and numerous applications. Since there is a lack of study of the initial formation and growing processes of condensed droplets down to nano-/submicroscale, relevant underlying mechanisms remain to be explored. We report an in situ observation of vapor condensation on nano-/microtextured superhydrophobic surfaces using optical microscopy. An interesting picture of the vapor condensation, from the initial appearance of individual small droplets (≤1 μm) to a Cassie-Baxter wetting state (>30 μm), are exhibited. It is found that individual droplets preferentially nucleate at the top and the edge of single micropillars with very high apparent contact angles on the nanotextures. Scenarios of two distinguished growing modes are reported statistically and the underlying mechanisms are discussed in the view of thermodynamics. We particularly reveal that the formation of the Cassie-Baxter wetting state is a result of a continuous coalescence of individual small droplets, in which the nanotexture-enhanced superhydrophobicity plays a crucial role. We envision that these fundamental findings can deepen our understanding of the nucleation and development of condensed droplets in nanoscale, so as to optimize design strategies of superhydrophobic materials for a broad range of water-harvesting and heat-transfer systems.
Highlights
This field of research has been very active for about twenty years, with theoretical, experimental and computational viewpoints
Rather than the concepts of the CB and partially wetted (PW) wetting states from a macro view14,43–48 – the diameter of the droplets are much larger than the size of individual textures, here we focus on the initial appearance and growing behaviors of very small droplets
It is interesting that two different growing modes at the beginning of the vapor condensation are observed: (1) When nucleation accidently happens on the top of individual micropillars, the droplet always attaches to the top during the initial growing process
Summary
This field of research has been very active for about twenty years, with theoretical, experimental and computational viewpoints. To the best of our knowledge, a systematic investigation of the initial condensation process of small droplets (i.e., ≤1 μm) on textures has yet to be done, and a fundamental understanding of their growth and development in natural environment remains extremely limited. In this context, it is worth bridging the wetting phenomena from the initial appearance of droplets in nano-/submicroscale and microscale to a formation of a Cassie-Baxter state in macroscale, to shed new light on dropwise condensation. This study will contribute to design optimal structures/materials with robust superhydrophobicity and a realization of a broad range of practical applications such as water collection and heat dissipation
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